inhibitory postsynaptic potential

An inhibitory postsynaptic potential, or IPSP, is a small dip in a neuron's inner voltage that makes the cell less likely to fire a signal. A neuron is a brain cell that talks to others by sending brief electrical pulses called spikes. Whether it fires depends on its voltage reaching a tipping point called threshold. An IPSP is the opposite of a push toward firing: it is a gentle pull in the wrong direction. Think of a dimmer switch being nudged down a little, so the light stays off.

Here is how it happens. When one neuron wants to quiet another, it releases a chemical messenger (often a molecule called GABA or glycine) at the junction between them, the synapse. That messenger opens tiny gateways in the receiving cell that let in negatively charged particles, or let positive ones leak out. Either way the inside of the cell becomes more negative than usual — a change called hyperpolarization, meaning it drifts even further from the threshold needed to fire. So the cell now needs extra excitation to reach that tipping point, which is why an IPSP holds the neuron back. Each IPSP is tiny, but many arriving together can add up and reliably keep a neuron silent, helping the brain stay balanced and sharp rather than firing wildly.